New and advanced transmission systems and braking systems are being developed by the automotive industry. These new systems often involve high energy requirements. Therefore, the friction materials technology must also be developed to meet the increasing energy requirements of these advanced systems.
The friction material must have high heat resistance in order to be useful in the new transmission and braking systems. Not only must the friction material remain stable at high temperatures, it must also be able to rapidly dissipate the high heat that is generated during operating conditions.
The high speeds generated during engagement and disengagement of the new transmission and braking systems mean that a friction material must be able to maintain a relatively constant frictional engagement throughout the engagement and disengagement of the friction material. It is important that the frictional engagement be relatively constant over a wide range of speeds and temperatures in order to minimize "grabbing" of materials during braking or "shuddering" of the transmission system during power shift from one gear to another.
In particular, the new high energy friction material must be able to withstand high speeds wherein surface speeds are up to about 14,000 feet/minute. Also, the friction material must be able to withstand high energy pressures up to about 700 psi. It is also important that the friction material be useful under limited lubrication conditions and also be able to withstand the extreme pressures and speeds applied during use.
Previously, asbestos fibers were included in friction materials. For example, the Arledter et al. U.S. Pat. No. 3,270,846 patent describe phenolic and phenolic-modified resins in asbestos-filled friction materials. Now, however, due to health and environmental problems, asbestos is no longer being used. However, friction materials without the presence of asbestos not only encountered structured integrity problems during processing, but also lacked the thermal stability which the asbestos provided. More recent friction materials have attempted to overcome the absence of the asbestos in the friction material by impregnating the paper or fiber materials with phenolic or phenolic-modified and other new resins. Attempts to overcome the limitations and drawbacks of phenolic resin impregnated friction materials include the replacement of phenolic resins with other thermosetting resins as the impregnating resin. One attempt to produce friction materials involves the modification of a phenolic resin with various synthetic resins, as described in the Takarada et al. U.S. Pat. No. 4,657,951. These friction materials, however, do not rapidly dissipate the high heat generated during use, which then contributes to reduced heat resistance and unsatisfactory coefficient of friction performance.
While phenolic resins are conventionally used as an impregnant in friction materials for "wet" applications, the phenolic resins have various limitations. The phenolic resin-impregnated friction materials do not have the high heat resistance necessary for use with the new high energy transmission systems. In particular, the phenolic resins in the impregnated friction material tend to carbonize at a temperature of about 450.degree. C. which is too low to be useful in high energy applications. In addition, phenolic resins are rigid materials and when the phenolic resins are used as an impregnant in a friction material, uneven lining wear and separator plate "hot spots" are more likely to result if uniform contact between friction materials is not obtained.
In the past, the friction materials were made by impregnating a friction paper with a suitable solvent-based impregnating resin and heating the impregnated paper to cure the resin. The solvent material must be evaporated off and collected in an environmentally sound manner. Further, during the evaporation of the solvent, the impregnating resin tends to migrate to the surface of the paper. This migration provides an uneven distribution of the impregnating resin in the friction paper material.
It is important that both the physical properties and the frictional characteristics of friction material remain intact during the expected service period of a friction material. A friction material impregnated with a brittle resin may crack under a heavy load and collapse the open structure of the friction paper lining matrix. On the other hand, a friction material impregnated with an elastomeric resin would provide desired friction torque, but lacks the wear resistance and the strength required to hold the friction paper matrix intact.
In order for friction materials to be useful, the friction material must have a wide variety of acceptable physical characteristics. The friction material must be resilient (or elastic) yet resistant to compression set, abrasion and stress, have high heat resistance and be able to dissipate heat quickly. The friction material must also have long lasting, stable and consistent frictional performance. If any of these characteristics are not present, optimum performance of the friction material is not met.
It is also important that the friction material be porous and possess uniform absorbency, and also have good tensile and shear strengths when saturated with brake lubricant or transmission fluid during use. In selected applications, it is also important that the friction material have a low density and high porosity such that there is a high fluid absorbency capacity during use. Thus, it is important that the friction material not only be porous, but also be flexible and compressible. The fluids absorbed into the friction material must be capable of being squeezed or released from the friction material quickly under the pressures applied during operation of the brake or transmission. It is also important that the friction material have high thermal conductivity to also help rapidly dissipate the heat generated during operation of the brake or transmission.
In view of the need for a better friction material, and as a result of extensive research, a new friction material with improved characteristics has now been developed. As far as is known, there is no disclosure of a friction material for use in "wet" applications wherein the friction material is made by adding a powdered phenolic resin and a powdered silicone resin (and, in certain embodiments, a silicon nitride) to a mixture of fibrous material to form a friction material.
Until the present invention, there has been no disclosure or suggestion that a powdered silicone resin and a powdered phenolic resin material (and, in certain embodiments, powdered silicon nitride) could successfully be blended with fibrous materials and other raw paper ingredients used in the slurry during the paper making process of friction materials. The friction material of the present invention eliminates the need for impregnating a porous fibrous base material with an impregnating resin.
Previous attempts to use silicone resins in friction materials have not had good acceptance in the friction lining industry. A friction lining that is impregnated or saturated with a silicone resin typically has poor shear strength and delamination resistance. Further, in the previously made friction linings the silicone resin tends to cause the friction lining to be too elastic which then creates undesirable friction characteristics. Therefore, it is not surprising that friction lining compositions formed entirely of a phenol-formaldehyde resin-polysiloxane resin have not been used successfully. Such compositions do not have the necessary constant coefficient of friction characteristics and thus fail under high energy and high heat conditions.
Until the present invention, there has been no disclosure or suggestion that powdered resins could be directly added to a raw paper formulation for use as a "wet" friction material. "Wet" friction materials are used in applications where the material is usually at least partially lubricated with the liquid used in wet brakes or an automatic transmission fluid, or other such media. During use of the "wet" friction material, the fluid is alternately squeezed from or is impregnating the friction material. Wet friction materials differ greatly in both their composition and physical characteristics from "dry" friction materials.
The surface chemical and mechanical interface tribological phenomenon of "wet" and "dry" materials contributing to performance are different. This refers to interface of friction material and steel separator plates which are in contact.
Accordingly, it is an object of the present invention to provide an improved friction material with reliable and improved properties compared to those of the prior art.
A further object of this invention is to provide friction materials with high thermal conductivity, porosity and strength.